Method and composition of platinum plating

ABSTRACT

A METHOD IS PROVIDED FOR THE ELECTROLESS PLATINUM PLATING ONTO METALS AND ALLOYS OF METALS MORE NOBLE THAN COPPER AND ONTO NON-CONDUCTIVE MATERIALS BY THE CHEMICAL REDUCTION OF PLATINUM FROM AN AQUEOUS ALKALINE SOLUTION CONTAINING A CHLOROPLATINATE COMPLEX.

United States Patent C) 3,698,939 METHOD AND COMPOSITION OF PLATINUM PLATING Frank H. Leaman, 1613 N. Harrison St, York, Pa. 17402 No Drawing. Filed July 9, 1970, Ser. No. 53,639 Int. Cl. C23c 3/02 US. Cl. 117-130 11 Claims ABSTRACT OF THE DISCLOSURE A method is provided for the electroless platinum plating onto metals and alloys of metals more noble than copper and onto non-conductive materials by the chemical reduction of platinum from an aqueous alkaline solution containing a chloroplatinate complex.

This invention relates to a process for electroless plating of platinum on various substrates which heretofore have not successfully been electrolessly platinum plated. More particularly, the process of this invention relates to the electroless plating of substrates from a plating solution containing a chloroplatinic complex of ammonium chloroplatinate.

According to this invention, substantial platinum deposits can be plated Without the extensive equipment necessary for electrodeposition techniques and an even deposit of platinum occurs in depressed areas and on parts too small or too complicated in design to plate by processes employing electrodeposition. The electroless platinum plating process also provides a method for the direct application of platinum to properly prepared and selected non-conductive materials, such as tetrafluoroethylene, polypropylene, polysulfone, etc., since these materials cannot be directly plated with platinum by electrolytic methods.

Prior attempts to directly plate non-conductive materials have been largely unsuccessful due to the requirements of high temperatures and corrosive acid solutions which are incompatible with many non-conductive materials. Also prior art coatings of these non-conductive materials are characterized by poor adhesion of the platinum to the non-conductive material.

Briefly, the process of this invention comprises contacting a properly prepared material with an alkaline electroless plating bath containing the chloroplatinate ion complexed With ammonia, hydrazine, chloride salts, wetting agents and water.

The main objective of the method of platinum plating in this invention is to obtain uniform deposits of platinum over noble metals in small depressions and over components too small or too complicated in design to plate evenly and efliciently by present electrolytic methods, and also to obtain deposits of platinum over non-conductive materials commonly plated by electroless processes. A dense buildup of platinum to thicknesses of 75 millionths of an inch is possible since the platinum deposited from the plating solutions used in this invention continuously catalyzes the deposition reaction until the solutions are depleted of platinum.

A further object of this invention is to provide an electroless method of platinum deposition in an auto-catalytic manner that enables the buildup of platinum over platinum previously deposited.

It is a still further object of this invention to provide an electroless method for depositing adherent, dense and continuous layers of platinum directly over metals more noble than copper and metal alloys more noble than copper to a predetermined thickness.

It is a further object of this invention to provide an electroless method for depositing adherent, dense and continuous layers of platinum directly over various non-conductive materials which have been plated by electroless techniques.

It is a further object of this invention to employ hydrazine or salts and derivatives of hydrazine, such as hydrazine sulfate, to reduce chloroplatinate complexed in ammoniacal aqueous solutions at a low temperature so as to obtain a dense metallic platinum deposit over substrates.

Still further objects and advantages of the present invention will become apparent from the following more detailed description of the invention.

The electroless plating composition used in the present invention is composed of various components characterized as follows:

STABILIZER The stabilizer employed in the plating bath of the invention is usually excess chloride ions. These ions keep the reduction of platinum under control during the reaction, and are supplied by permitting a trace of free chloride ion in the chloroplatinic acid salt that is used to prepare the bath or by adding Cl ion containing salts such as NH Cl, NaCl, KCl, etc. The stability of the chloroplatinate complex in ammoniacal solutions increases with an increase in solution pH, so NH OH can also serve as a stabilizing agent. The proper balance of these stabilizing agents permits the platinum to plate out at a controlled rate.

DRIVING FORCE The activator of the solution is the chemical reducer. l-lydrazine and hydrazine sulfate are preferred for this purpose, however other hydrazine compounds, such as hydrazine dihydrochloride, etc. may also be used as reducing agents.

PLATING MATERIAL Chloroplatinate complexed with ammonia supplies the platinum to be reduced by the driving force to form pure platinum deposits on the object to be plated. The chloroplatinate complex, (NH PtCl is prepared as follows:

Pure platinum sponge is dissolved in aqua-regia. The acid solution is then evaporated to dryness over a low heat followed by repeated dissolution of the chloroplatinic acid residue in distilled water and evaporation to dryness until only a trace of free chloride ion remains in the residue. The chloroplatinic acid salt is dissolved in distilled Water to the desired concentration. This solution may be stored indefinitely. Just prior to use, a given amount of chloroplatinic acid solution is mixed with a solution of ailnmonium hydroxide to form the chloroplatinate comp ex.

WETTING AGENT The wetting agent evens out the plated coating, i.e., it promotes a uniform thickness of pure platinum metal on the entire surface of the object to be plated. The wetting agent also assures even deposition in the small depressions of complicated pieces. Suitable wetting agents are non- Constituent: Moles per liter Platinum (ammonium chloroplatinate) 0.00050.15

Hydrazine or hydrazine salts 0.005-1.50 Ammonium hydroxide 0.005-13.0 Ammonium or alkali metal chlorides 0.005l.0 Wetting agents (surfactants) -0.10

In addition to the main bath constituents set forth above, various other salts may be added to improve plating efiiciency, to enhance grain refinement in the deposits, to lower the solution surface tension and aid in the release of nitrogen evolved during plating and to reduce the degree of stress in the deposited metal. These compounds include various salts such as 8-amino-1,5-naphthalenedisulfonic acid disodium salt, 2,7-naphthalenedisulfonic acid, p-amino-benzenesulfonic acid, benzenesulfonic acid, etc. These compounds are usually added at concentrations of up to 0.10 mole per liter.

The total reaction for the chemical deposition of platinum is illustrated as follows:

Half reactions showing electron exchange are as follows:

The reaction for the formation of the chloroplatinate complex is as follows:

wherein X has the same meaning as above.

Examination of the stoichiometric equations representing the electroless platinum plating solutions herein described shows these solutions require a precise balance between the chloroplatinate complex concentration, the hydrazine concentration, the chloride ion concentration and the ammonium hydroxide concentration. The initial excess of chloride ions prevents the spontaneous decomposition of the bath by keeping the half reactions in Equations 2 and 3 from taking place too rapidly.

Heat also drives the reaction to completion since the reduction is temperature dependent. The solutions in this reaction are efiicient over a temperature range of from C. to 100 C. Temperatures at the upper end of the allowable temperature drive the reaction faster and the temperature must be carefully controlled to avoid reducing the platinum too quickly forming uneven deposits and poor adherence. For these reasons, it is generally advantageous to immerse the parts to be plated in the plating solution at room temperature within several minutes after the preparation of the bath and gradually increase the solution temperature during plating to ensure exhaustion of the platinum.

Other means of easing the deposition of platinum are decreasing the platinum complex concentration, decreasing the chloride ion concentration, decreasing the ammonium or alkali metal chloride concentration or by increasing the hydrazine concentration. Although ammonium hydroxide is normally added to these solutions in excess, a decrease in their concentration will also increase the rate of platinum deposition. A proper balance of these constituents will provide a plating solution of nearly 100% efiiciency.

Since the plating bath is stable and allows the platinum to plate slowly, dense, thick and adherent coatings of platinum may be obtained in one plating step instead of many as are required by less stable plating solutions. A high platinum concentration may advantageously be employed with an increase in bath plating temperature to efficiently plate materials with coating of platinum up to 75 millionths of an inch.

The plating solution is mixed in two separate solutions: the chloroplatinate complex solution and the hydrazine solution. Just prior to use the chloroplatinate acid solution is mixed with the proper amount of ammonium hydroxide. The hydrazine solution should be prepared just prior to use because the hydrazine hydrolyzes slowly in aqueous solutions. Plating efficiency is reduced if the hydrazine becomes hydrolyzed prior to bath preparation. All other components of the final plating bath should also be added to the hydrazine solution. Just prior to plating, the chloroplatinate complex solution and the hydrazine reducer solution are mixed to prepare the active electroless platinum plating bath.

The process of this invention is useful for electroless plating on metals and metal alloys more noble than copper such as copper, silver, nickel, palladium, gold, copper silver alloys, etc., and also on non-conductive materials such as acrylonitrile-butadiene-styrene plastic, tetrafluoroethylene, polysulfone, polypropylene, acrylic plastics, aluminum oxides, glasses, etc.

The following examples illustrate the compositions useful in this invention and the plating method of this invention. These examples are illustrative of various embodiments of this invention and are not to be construed as limitative.

Example 1 A stock platinum solution was prepared at a concentration of 1 gram per liter platinum from chloroplatinic acid salt that was stabilized by successive evaporations with distilled water to expel free chloride ion. This solution was neutralized with several drops of ammonium hydroxide solution.

A piece of Lustran ABS 262 foil 0.6" x 1.0 was prepared for plating by cleaning in caustic solution, water rinsing soaking in a warm nitric and sulfuric acid-wetting agent solution (Enplate 470 conditioner) 30 minutes, water rinsing, immersing in an acidic stannous chloride wetting agent solution (Enplate 432 sensitizer solution) 2 minutes, water rinsing, immersing in an acidic palladium chloride solution, Enplate 440M activator solution) 2 minutes, and water rinsing. The foil was placed in a nearly neutral 0.25 gram per liter platinum bath that was pre pared by pouring solution No. 2 into solution No. l as follows:

Solution No. 1:

'5 cc. chloroplatinic acid solution neutralized as above' 5 cc. distilled Water Solution No. 2:

10 cc. distilled water 1 drop 3% hydrazine solution 1 gram ammonium chloride The sample of ABS foil was plated 180 minutes at room temperature. During plating, the bath pH was maintained at pH 7.0-pH 7.3. A platinum deposit of 0.00233 gram was plated on the sample. Bath efliciency was 46.5%.

Example 2 A plate of Lustran ABS 640 plastic 0.10" x 3.0" x 4.0" was prepared for plating by the procedure given in Example 1. The plate was placed in a 0.25 gram per liter platinum plating solution that was prepared by pouring solution No. 2 into solution No. 1 at room temperature.

Solution No. 1:

12.5 cc. chloroplatinic acid solution (per Example 1) cc. ammonium hydroxide Solution No. 2:

287 cc. distilled water 30 drops 50% hydrazine solution The bath was gradually warmed to 72 C. in 35 minutes, then the sample was removed from the bath. A bright reflective deposit of 0.0120 gram platinum was plated on the sample. Bath efliciency was 96%. The deposit thickness was about 12 millionths of an inch.

Example 3 A stock platinum solution was prepared at a concentration of grams per liter platinum from chloroplatinic acid salt that was treated by successive evaporations with distilled water to expel free chloride ion.

Example 4 Twenty acrylic plastic components having a total surface area of 8.2 in. were plated in a 4 gram per liter platinum plating solution that was prepared by pouring solution 'No. 2 into solution No. 1 at room temperature.

Solution No. 1:

20 cc. chloroplatinic acid solution 10 cc. ammonium hydroxide Solution No. 2:

20 cc. distilled water 6 drops 50% hydrazine solution The bath was warmed to 70 C. in 6 minutes in a hot water tank. Deposition of platinum began at 70 C. and was continued for 20 minutes to a final temperature of 80C. An attractive, adherent deposit of platinum was plated on the components. Bath elficiency was 90.5% and the deposit thickness was about 45 millionths of an inch.

Example 5 A piece of Lustran ABS 262 foil 0.5"x 1.5" was prepared for plating by the procedure given in Example 1. The foil was placed in a 1 gram per liter platinum plating solution that was prepared by pouring solution No. 2 into solution No. 1 at room temperature.

Solution No. 1:

5 cc. chloroplatinic acid solution (per Example 3) 10 cc. ammonium hydroxide Solution No. 2:

35 cc. distilled water 4 drops 50% hydrazine solution The bath was gradually heated to 83 C. over a 100 minute period. Initial deposition of platinum occurred at 33 C. A deposit of 0.03815 gram platinum plated on the sample. Bath efiiciency was 76.3% and the deposit thickness was about 75 millionths of an inch.

Example 6 A bath prepared by the procedure given in Example 5 was used to plate an 18K gold 1.0" x 1.5" foil. The bath was heated during a period of 25 minutes to 80 C. Bath efliciency was 70% and the deposit thickness was 35 millionths of an inch. The deposit was very adherent and it was decorative in appearance.

Example 7 A piece of 10 mil Avisuns grade 12-270 A polypropylene foil 15 x 2.0" was prepared for plating by precleaning the polypropylene in a caustic solution, followed by 2 cold water rinses, conditioning the polypropylene by immersing in a solution of a mixture of phosphoric and sulfuric acids with wetting agents saturated with chromic acid (Enplate G-DO37), at 190 F. for 12 minutes. After thorough rinsing with cold water, the polypropylene was immersed in a solution of 15 parts by volume of deionized water and 1 part by volume of an acidic palladium chloride solution (Enplate 440M activator), for 3 minutes at 135 F. After rinsing twice with cold water, the polypropylene was ready for electroless plating. The foil was plated in a 1 gram per liter platinum bath prepared by pouring solution No. 2 into solution No. 1 as follows. The reducer solution was filtered through a double retention filter prior to mixing the bath.

Solution No. 1:

:10 cc. chloroplatinic acid solution (per Example 3) 20 cc. ammonium hydroxide Solution No. 2:

70 cc. distilled water 0.04 gram 8-amino 1,5 naphthalenedisulfonic acid disodium salt 0.04 gram 2,7-naphthalene disulfonic acid 1.30 grams hydrazine sulfate 8 drops 5% nonyl phenol tergitol NPX solution.

The sample was plated 5 minutes at room temperature, then the bath was gradually heated over a period of 25 minutes to 75 C. A weight of 0.0582 gram platinum was deposited on the sample. Bath efficiency was 58.2% and the deposit thickness was about 25 millionths of an inch.

Example 8 The plating trial of Example 7 was similarly repeated except for the substitution of 10 drops 50% hydrazine solution in place of the hydrazine sulfate.

A deposit of 0.0820 gram platinum was deposited on the sample. Bath efiiciency was 8 2% and the deposit thickness was about 40 millionths of an inch.

Example 9 Two 0.10" x 1.0" X 2.5" Union Carbide P-1700 polysulfone samples and two 0.10" x 1.0" x 2.5" Union Carbide P3510 polysulfone samples were sandblasted lightly and were conditioned for plating by immersing the polysulfone in an organic liquid solvent ('Enplate SC-46l) at room temperature for 3 to 10 minutes followed by two water rinses. The polysulfone was further conditioned by immersing in a nitric and sulfuric acid-wetting agent solution (Enplate Conditioner 470), for 3 minutes at 160 F. followed by two cold water rinses. The polysulfone was then sensitized by immersing in a solution of '15 parts by volume of deionized water and one part by volume of an acidic stannous chloride wetting agent so lution (Enplate Sensit-izer 432), at room temperature for one minute followed by two cold water rinses. The polysulfone was then activated in a solution of 15 parts by volume of deionized water and one part by volume of an acidic palladium chloride solution (Enplate Activator 440M) followed by two cold water rinses. These samples were plated in the following bath prepared by pouring solution No. 2 into solution No. 1 at room temperature.

Solution No. l:

15 cc. chloroplatinic acid (per Example 3) 30 cc. ammonium hydroxide Solution No. 2:

105 cc. distilled water 0.06 gram p-aminobenzenesulfonic acid 0.06 gram 8-arnino-l,5-naphthalenedisulfonic acid disodium salt 12 drops 5% nonyl phenol tergitol NPX solution '15 drops 50% hydrazine solution.

The samples were plated 5 minutes at room temperature, then the bath was slowly heated to 70 C. in 25 minutes for a total plating time of 30 minutes. The platinum deposit was very adherent and conductive on each sample. The deposits were about 20 millionths of an inch in depth.

'While certain desired embodiments have been illustrated by way of specific example, it is to be understood that the present invention is not limited thereto but should be construed to embrace any and all equivalents thereof.

I claim:

1. A method of electroless deposition of platinum which comprises contacting at a temperature in the range of from 20 C. to C. a material, selected from the group consisting of a metal more noble than copper, and non-conductive materials, with an aqueous electroless plating solution consisting essentially of from 0.0005 to 0.15 mole per liter of a chloroplatinate acid salt complexed With ammonium hydroxide, from 0.005 to 1.50 moles per liter of a platinum reducing agent selected from the group consisting of hydrazine and salts of hydrazine, and from 0.005 to 13.0 moles per liter of NH OH, said aqueous electroless plating solution having a pH in the range from pH 7.0 to pH 13.5.

2. The method of claim 1 wherein the reducing agent is hydrazine.

3. The method of claim 2 wherein the material to be plated is acrylonitrile-butadiene-styrene plastic.

4. The method of claim 2 wherein the material to be plated is acrylic plastic.

5. The method of claim 2 wherein the material to be plated is gold.

6. The method of claim 2 wherein the material to be plated is polysulfone.

7. The method of claim 2 wherein the material to be plated is polypropylene.

8. The method of claim 1 wherein said plating solution is formed by mixing said reducing agent into said chloroplatinate salt-ammonium hydroxide complex while at room temperature.

9. A method of electroless deposition of platinum which comprises contacting at a temperature in the range of from 20 C. to 100 C. a material, selected from the group consisting of a metal more noble than copper and nonconductive materials, with an aqueous electroless plating solution comprising from 0.0005 to 0.15 mole per liter of a chloroplatinate acid salt complexed with ammonium hydroxide, from 0.005 to 1.50 moles per liter of a platinum reducing agent selected from the group consisting of hydrazine and salts of hydrazine, less than 0.10 mole per liter of a wetting agent selected from the group consisting of nonyl phenol, alkyl naphthalene sulfonate, alkyl aryl sulfonate, alkyl phenoxy polyoxyethylene ethanol and mixtures thereof, and from 0.005 to 13.0 moles per liter of 8 NH OH, said aqueous electroless plating solution having a pH in the range from pH 7.0 to pH 13.5.

10. A method of electroless deposition of platinum which comprises contacting at a temperature in the range of from 20 C. to C. a material, selected from the group consisting of a metal more noble than copper and non-conductive materials, with an aqueous electroless plating solution comprising from 0.0005 to 0.15 mole per liter of a chloroplatinate acid salt complexed with ammonium hydroxide, from 0.005 to 1.50 moles per liter of a platinum reducing agent selected from the group consisting of hydrazine and salts of hydrazine, from 0.001 to 1.0 mole per liter of a chloride salt selected from the group consisting of NH Cl, NaCl, KCl and mixtures thereof, and from 0.005 to 13.0 moles per liter of NH OH, said aqueous electroless plating solution having a pH in the range from pH 7.0 to pH 13.5.

11. The method of claim 9 wherein the aqueous electroless plating solution contains a total concentration of less than 0.10 mole per liter of a compound selected from the group consisting of 8-amino-1,5-naphthalene disulfonic acid disodium salt, p-aminobenzene sulfonic acid, 2,7-naphthalenedisu1fonic acid, benzenesulfonic acid and mixtures thereof.

References Cited UNITED STATES PATENTS 2,915,406 12/1959 Rhoda et a1. 106-1 3,300,335 1/1967 Hol'vath et al. 117----130 3,423,228 1/1969 Oster et al. 117-47 3,486,928 12/1969 Rhoda et a1. 117-130 FOREIGN PATENTS 872,785 7/ 1961 Great Britain 1061 LORENZO B. HAY-ES, Primary Examiner US. Cl. X.R. 

